Low power lasers are employed to read and write binary data on the data side of media. One typical media are optical storage discs, such as CDs, DVDs and the like. Typically, various types of data are written on the data side of the disc by a laser beam while the disc is rotating. Data may be recorded by changing a property of a desired area on the recording media so that the area is indicative of a zero or one data value. Various data writing strategies have been employed, for the purposes of creating desirably shaped marks precisely positioned and having sharp edges to enable detection. Data is typically both written and read using the same laser, although the power used to write data is typically higher than the power used to read data.
Non-data areas such as the side of a data disc opposite the data side is often used for handwriting or affixing or marking a label with descriptions and illustrations corresponding to the recorded data. Recently, apparatus and methods have been developed with the ability to generate an optically-visible label on the non-data region of an optical disc using the same laser that was employed to read and write digital or electronic data on the data side of the disc. See U.S. Patent Application Publication No. 2003/0108708 (Anderson, et al.), disclosing the use of laser sensitive materials on a disc label that react chemically with the application of light and heat and result in changes in color and shading on the label.
In order to form labels having high image quality, the marks should have a good quality of darkness or optical density. The range of laser power that produces acceptable marking is relatively narrow, and power variations that may occur in the laser apparatus requires that the laser power be calibrated to bring the power within an acceptable margin of variation. Accordingly, methods of measuring marks are needed that will insure that the marks have acceptable quality. Measuring the marks using a laser in the drive would be advantageous.
However, it can often be difficult to detect with the laser optically visible marks that have been made in the light sensitive material by the laser. The chemistry of the label is optimized to absorb as much light as possible at the wavelength of the writing laser beam. As a result, there is very little reflectivity of light at the wavelength of the writing laser, and thus the change in reflectivity between written and unwritten areas is very small. Accordingly, it has been difficult to detect the quality of the visible marks being made on the label and to improve or optimize the marks, because of a very low signal-to-noise ratio (SNR). Other factors contributing to the low SNR problem are (1) the surface texture of the recording material, (2) noise in the photo-detectors generating the detection signal, (3) ambient light striking the photo-detectors, (4) noise in the read channel of the detection signal, and (5) high sensitivity to changes in reflectivity at different wavelengths.